1. Field of the Invention
This invention relates to valves used in subterranean oil or gas wells which are actuated by control fluid pressure.
2. Description of the Prior Art
Valves for controlling the flow of fluids in a subterranean oil and gas well are employed both at the surface of the well and often within the well itself. Perhaps the most common type of downhole valve is the surface control well safety valve. Conventionally these valves are actuated by an increase or decrease in the control fluid pressure in a separate control line extending from the valve to the surface of the well. Although there are numerous types of downhole safety valves the most common valves are ball valves and flapper type safety valves. Conventional ball type safety valves employ a rotatable spherical head or ball having a central flow passage which can be aligned with respect to the bore of the valve to permit flow through the valve and therefore in the fluid transmission conduit or tubing string. Rotation of the ball valve element through an angle of 90.degree. will prevent flow through the central flow passage and will close the valve. Rotation of ball valve elements between open and closed positions is generally imparted by a combination of longitudinal and rotary movement of the valve. Conventionally a camming element or actuating mechanism is used to rotate the ball valve head during axial movement of the valve. Axial movement is generally imparted to the valve by increasing the control pressure acting in the control line and acting on a pressure movable member within the valve itself. As pressure is increased a sleeve or piston is generally moved in one axial direction which in turn will cause the valve to rotate from a closed to an open position permitting flow through the valve. A reduction in control pressure, which may be caused by a rupture in the control line or destruction of the wellhead, will permit the valve to automatically close.
Another common type of surface control well safety valve employs a flapper valve head. Rotation of the flapper about an axis transverse to the axis of the valve bore will open the valve bore. The conventional means of actuating the valve head is to employ an axially movable flow tube which abuts the downstream surface of the flapper head and causes the flapper head to rotate about its hinge to open the valve bore. Normally axial movement of the flow tube is caused by an increase in control fluid pressure acting on one surface of the flow tube or on one surface of a member interconnected with the flow tube. As with the ball type safety valve a reduction in control fluid pressure will permit axial movement of the flow tube in the opposite direction and will allow the flapper valve head to close the valve bore.
There are certain limitations on the performance of conventional ball type or flapper type safety valve. For example hostile environmental conditions such as temperatures in access of 500.degree. or the presence of toxic liquids and gases can adversely affect the performance of elastomeric sealing elements used in conventional safety valves. There is also a limitation upon the depth at which conventional safety valves can be employed. The hydrostatic pressure of the control fluid in the external control pressure line must always be less than the actuating pressure necessary to open the safety valve. Thus positioning conventional safety valves at great depths would require the use of quite large springs to resist movement of the control actuating sleeves or pistons. One safety valve design which has been proposed as a means for overcoming the depth limitations placed upon conventional safety valve is a piston actuated safety valve disclosed in U.S. Pat. No. 4,161,219. This valve employs a separate piston located in the valve housing to actuate a cylindrical flow tube or sleeve which in turn is used to open a conventional flapper valve head. The cross-sectional area of the piston is less than the cross-sectional area of the flow tube thus resulting in a smaller hydrostatic pressure force tending to open a deep set safety valve. This valve still employs conventional elastomeric sealing elements on the piston member. Elastomeric sealing elements are not only adversely affected by hostile conditions in the well but are unsuitable for use with gases. Therefore liquids must be used as the control fluid, despite the fact that pressurized gas is often conveniently available at the well site. The elimination of elastomeric seals in the control pressure actuating system would permit the use of gas as a control fluid and would ease the environmental limitations on the use of safety valves.